One such apparatus is disclosed in DE 43 33 614 A1. This apparatus consists of a spreading station, steam moistening apparatus, preheating section, and a continuously operating press, these four apparatuses being joined together in a continuously running and circulating manner by an endless woven metal belt having in each of its two marginal areas a heat-resistant plastic composition, for example Teflon.
The problem that was presented was that the press factor, especially in the processing of long shavings spread with orientation (resulting in OSB boards), is substantially greater than in the production of chip board. In addition to the negative influence of the coarse chip structure the poor press factor was due to the following: the processing of pressed boards of wood material, such as chip board, MDF (medium density fiber board), or OSB boards is performed technologically according to the principles that the wood particles—in this case the large-area oriented shavings for the OSB boards—are wetted with a moist fluid resin content (for example, phenolic resin binders), and that this water is evaporated when the chip mat in the press is heated, and the formation of steam, especially in the core of the boards being manufactured, produces a surrounding field of heat that is equal to or greater than 100° C. Since in the normal production of chip boards or MDF boards, the chip mat is enclosed between smooth press surfaces (hot plates or steel belts), a pressure higher than 1 bar can form between the large-area press zones. According to the steam pressure diagram, the temperature then rises with the rising steam pressure. In general, a temperature level of about 120° C. establishes itself between the upper and lower press surfaces. Due to the steam pressures in excess of 1 bar, an accelerated transfer of steam occurs from the outer layers to the middle layers, which results in an accelerated curing, especially in the core of the boards. This elevated steam pressure cannot establish itself through the metal mesh belt, because the mesh belt does not permit any build-up of pressure, so that only a wet steam is formed in the range around about 100° C., so that an accelerated curing in the core of the board is not possible. Ultimately, this results in the press factors that are approximately twice as high than in any normal production of chip boards.
For the reasons set forth, the production of OSB boards was economical only on a multiple-day apparatus with a very great number of stages. For the same reason, the use of continuously operating presses has hardly established itself in the production of OSB, because due to the high press factor, excessively long presses would have to be used, which would require an excessively high capital investment in proportion to productivity. On the other hand, however, the manufactured homes industry requires OSB boards in which at least one side displays a surface texture in the form of a mesh belt impression made by a metal wire mesh. The metal wire mesh serves in multi-stage presses for the transport of the coarse wood chips which are spread onto the metal wire mesh belt, and which cannot be pre-compressed in a fore-press. On the other hand, it provides for the surface texture on the pressed OSB boards which is functionally necessary for further processing.
With the method and apparatus according to DE 43 33 614 A1 it has been possible to improve the press factor to such an extent that an economical manufacture of chip boards can be achieved in a continuously operating press from a chip mat with large-area, oriented long shavings.
In the implementation of the invention according to DE 43 33 614 A1, it has developed that the method and the apparatus are suitable for the production of OSB in fast pressing time. The method and the apparatus, however, are capable of improvement, namely in regard to reducing the press time, the quality of the surface texture created, and the quality of the board.
An apparatus has furthermore been disclosed in DE 197 04 643 C2 in which, in the continuously operating press for the manufacture of primarily OSB boards, a circulating mesh belt is also carried through the press. In this apparatus the attempt has been made to prevent thermal expansions and differential expansions from resulting in damage to the steel belt and/or the mesh belt. The invention attempts to prevent damage by using a mesh belt and steel belt made of materials with equal thermal expansion properties, and by various measures to equalize their temperatures before they run into the continuously operating press. Thus relative movements between the steel belt and mesh belt are said to be prevented. But the steel belt and mesh belt have a very low thermal conductivity since they are made of high-alloy stainless steel. It has turned out that equipment of this kind has a production rating about 5% lower if the steel and metal mesh belts consist of high-alloy steels. The problem is that the heat has to be carried over the heating plates, through the steel belt and through the metal mesh belt to the surface of the material being pressed. The heat flow is hampered by the low thermal conductivity of the metal mesh belt. This reduced heat flow results in a slower heating of the material mat, especially in the center of the mat, within the continuously operating press, and thus results in longer press time and slower steel belt running and production rates.